1. Field of the Invention
This invention relates generally to the field of human physiology, and, more particularly, to methods, apparatus, systems, and computer program products for coordinating musculoskeletal and cardiovascular hemodynamics.
2. Related Art
Blood is circulated through the body by the heart during its rhythmic pumping cycle, which consists of two distinct periods—systole and diastole. Heart muscle (myocardium) contracts to eject blood from the ventricles during the systolic period of each cardiac cycle (CC). Ejection of blood from the ventricles generates arterial blood pressure and flow adequate to deliver blood throughout the body, thereby transporting oxygen, nutrients and metabolic products, removing carbon dioxide and waste, and facilitating critical physiological functions such as heat exchange. The heart subsequently relaxes during the diastolic period of the CC, when the atrial and ventricular chambers refill with blood in preparation for the heart's next contraction.
Unlike the rest of the body, which receives most of its blood flow as a result of pressure generated during systole, the heart's own arterial blood supply is delivered primarily during the diastolic portion of the cycle when the heart muscle is relaxing and the heart chambers are filling for the next contraction. Little blood flows to perfuse the myocardium during systole because the heart's contraction generates high forces within its muscular walls and thereby prevents flow through the coronary blood vessels that travel across and through the myocardium. During diastole, when the heart muscle has relaxed, residual blood pressure in the aorta drives blood flow through the coronary arteries and into the myocardial muscle, supplying the heart with its needed oxygen and nutrients.
In addition to the heart's pumping function, the musculoskeletal (MSK) system also plays an important role in circulating blood throughout the body during physical activity. Arterial and venous blood is pumped rhythmically throughout the body via transient changes in peripheral vascular pressure induced by many types of repetitive MSK activities. Skeletal muscle contraction and relaxation cycles during rhythmic physical activities cause regular oscillations in peripheral arterial and venous blood pressure or flow due to intermittent compression of the vasculature, while MSK movement can lead to periodic acceleration and deceleration of the intravascular volume of blood against gravity and inertia.
When rhythmic muscle contractions and MSK movements are favorably coordinated with the heart's pump cycle, the two pumping systems can augment one another, thereby increasing blood flow and perfusion to important areas of the body with less pumping energy expended by the heart. This favorable coordination of the two pumping systems can be referred to as “musculoskeletal counterpulsation” (MCP). During MCP, maximum rhythmic MSK-induced blood pumping consistently occurs while the heart is relaxing and refilling between contractions, and the maximum cardiac induced pumping consistently occurs between MSK maximal pumping events. On the other hand, when rhythmic muscle contractions and MSK movements occur with uncoordinated, or worse, unfavorably coordinated timing, blood flow and perfusion are decreased along with a concurrent decrease in pumping efficiencies. Unfavorable coordination occurs, for example, when the CV and MSK systems consistently pump blood maximally into the central circulation at substantially the same time during rhythmic physical activity. This unfavorable coordination of the two pumping systems can be referred to as “inverse musculoskeletal counterpulsation” (iMCP).
Typically, when individuals walk, run, bicycle, or participate in any rhythmic physical activity, most experience favorable coordination between MSK blood pumping and CV blood pumping only occasionally. Even when an individual's heart rate (HR) and exercise cadence happen to be equal, the respective timing of the two pumps may result in favorable or unfavorable coordination, or somewhere in between. Research has shown that a certain degree of “cardio-locomotor synchronization” can occur during rhythmic physical activity, in which the timing of an individual's MSK pump cycle relative to their heart's pump cycle tends, statistically, to naturally favor MCP. However, when such synchrony does occur, it is usually only a temporary phenomenon since HR and/or cadence can change as environmental factors vary (e.g., running in hilly terrain or variable wind), or with any of several physical changes, such as alterations in effort or speed, hydration, temperature, catecholamine levels or fatigue.
The benefits of favorable coordination between MSK movements and the heart's pump cycle can include improved perfusion and oxygenation of cardiac and peripheral skeletal muscle and possibly other tissues; a lower heart rate (HR) due to increased cardiac preload and stroke volume; a decrease in systolic blood pressure and pulse pressure; a decrease in required respiratory effort to meet the decreased oxygen demands; less muscle fatigue due to improved skeletal muscle perfusion. All of these benefits can combine to result in increased physiological efficiency, decreased myocardial stress, increased aerobic energy production capabilities and improved potential for aerobic fat metabolism, enhanced individual performance, and a potential increase in the health benefits and safety of rhythmic physical activity. Conversely, lack of coordination or unfavorable coordination between MSK movements and the heart's pump cycle can lead to the opposite of all of these effects.
As an individual's level of physical activity increases, the typical healthy heart increases its rate of pumping in response to the increased metabolic demands generated by the intensity of the action. In some hearts, this chronotropic capability is compromised and the individuals are said to be chronotropically incompetent. As a result, the individual faces symptoms that include shortness of breath during activities of modest intensity, which impairs quality-of-life. Individuals suffering from chronotropic incompetence are typically treated with an implanted rate-responsive pacemaker that stimulates the heart at a rate commensurate with the intensity of the activity. Pacemakers can use different mechanisms to determine rate responsiveness for a specific intensity of activity. Also, several mechanisms exist to measure the intensity of activity.
The earliest pacemakers were not rate responsive and had only the capability to provide stimulation pulses to the heart at a fixed cardiac pacing rate. A patient could feel wide-awake when attempting to sleep or exhausted while attempting to exercise because their heart was beating at a steady rate that might be too high for comfortable resting but too low to meet the metabolic demands of many levels of physical activity.
To address problems with fixed-rate pacemakers, numerous methods have been used to adjust the pacing signals to the heart in response to the patient's immediate need. Such methods include accelerometry to sense the level of patient activity; thoracic impedance changes to reflect minute ventilation; temperature measurements as indicators of central venous temperature; QT sensors for measuring QT interval variations (a metric on the electrocardiogram/electromyogram). QT sensors are much better metabolic sensors and QT interval variations are a function of the intensity of activity and circulating catecholamine in the blood stream. Consequently, QT sensors are highly specific to exercise and post-exercise recovery as well as mental stress. Additionally, sensors capable of measuring physiologic responses, such as changes in blood pressure, blood oxygen content, pulse rate, blood flow, or myocardial or endocardial tissue acceleration, may also be used in conjunction with any of the above mentioned rate response sensors, to get more specific information about intra cardiac activity and to regulate the HR by appropriately timing the stimulating pulse from the pacemaker.
Each of these prior pacemaker rate-adjusting methods comes with their respective advantages and limitations. Nonetheless, adapting the pacing rate in response to one or more such sensing modalities offers advantages over the earlier non-rate responsive devices. None of these approaches however has attempted to coordinate the timing of the heart's pump cycle with the patient's repetitive physical activities.
Cardiac exercise stress testing is an important diagnostic modality that typically tests cardiac function during rhythmic physical activity (e.g. treadmill walking and running, and bicycle exercising). These tests are plagued by frequent false positive results. Uncontrolled rhythmic MSK activity that matches the patient's HR during the observation period may influence the apparent results, unbeknownst to the clinician performing the analysis.